This invention relates to a process for producing small or minute capsules containing a water-soluble material which comprises dissolving a first shell wall component in the water-soluble material, which is the material to be encapsulated, dispersing the resulting mixture, said mixture being the aqueous or discontinuous phase liquid into an organic or continuous phase liquid containing an emulsifier which is an oil soluble alkylated polyvinylpyrrolidone polymer to form a water-in-oil (W/O) emulsion and thereafter adding a second shell wall component (usually dissolved in additional oil phase liquid) to the water-in-oil emulsion whereby the second shell wall component reacts with the first shell wall component to form a polymeric shell wall about the water-soluble material at the water/oil interface.
The process of microencapsulation described herein is a modification of known interfacial polycondensation techniques. Such techniques are thoroughly described in the literature. An article entitled "Interfacial Polycondensation, a Versatile Method of Polymer Preparation" by P. W. Morgan, Society Plastics Engineers Journal 15, 485-495 (1959), provides a good summary of the reactions involved and the polymers which can be used in this method. The use of the technique of interfacial polymerization in a process of microencapsulation is also known; e.g., MICROCAPSULE PROCESSING AND TECHNOLOGY, Asaji Kondo, Edited by J. Wade Von Valkenburg, pp. 35-45, Marcel Dekker, Inc., New York, NY 10016 (1979). Exemplary of the patents directed to microencapsulation of water-soluble materials via interfacial polycondensation reaction are U.S. Pat. Nos. 3,429,827, 3,577,515, 3,575,882 and 4,251,387.
Microencapsulation of water-soluble materials utilizing an interfacial polycondensation reaction generally involves the following procedure. A first reactive monomeric or polymeric material(s) (first shell wall component) is dissolved in the material to be encapsulated to form the aqueous or discontinuous phase liquid. The discontinuous phase liquid is dispersed into an oily (organic) or continuous phase liquid to form a water-in-oil (W/O) emulsion. The continuous phase (organic) liquid may contain a second reactive monomeric or polymeric material (second shell wall component) at the time the discontinuous phase is dispersed into the continuous phase. If this is the case, the first and second shell wall components will immediately begin to react to form a polycondensate shell wall about the material to be encapsulated. However, the preferred practice is to form the W/O emulsion before the second shell wall component is added to the emulsion. This enhances the formation of a stable W/O emulsion before the interfacial polycondensation reaction is initiated.
The capsules produced in this fashion may be any desired size, for example, of the order of 1 micron up to 100 microns or larger in diameter, preferably the size of the microcapsules will range from about 1 to about 50 microns in diameter. Capsules of this character have a variety of uses, as for containing water-soluble dyes, inks, chemical agents, pharmaceuticals, flavoring materials, water-soluble pesticides, e.g., herbicides, plant growth regulants, insecticides, fungicides, and the like. Any water-soluble material into which the first shell wall component can be dissolved and which is nonreactive with said first shell wall component may be encapsulated with this process. Once encapsulated, the liquid or other form is preserved until it is released by some means or instrumentality that breaks, crushes, melts, dissolves, or otherwise removes the capsule skin or until release by diffusion is effected under suitable conditions.
A method of encapsulating water-soluble materials by interfacial condensation between directacting, complimentary reactants is disclosed in U.S. Pat. No. 3,577,515, which describes a method which requires a first reactant (shell wall component) and a second reactant (shell wall component) complimentary to the first reactant, with each reactant in separate phases, such that the first and second reactants react to form encapsulated droplets of water-soluble or water-immiscible material. The process is applicable to a variety of polycondensation reactions, i.e., to many different pairs of reactants capable of interfacial condensation from respective carrier liquids to yield solid film at the liquid interface. The resulting capsule skin may be produced as a polyamide, polysulfonamide, polyester, polycarbonate, polyurethane, polyurea or mixtures of reactants in one or both phases so as to yield corresponding condensation copolymers.
Although it is known in the art, e.g., U.S. Pat. No. 3,464,926 and U.S. Pat. No. 3,577,515, that microencapsulation of water-soluble materials is possible using well known emulsifiers, e.g. lecithin or "Span 60", to prepare water-in-oil emulsions, emulsions made with these common emulsifiers do not support shell wall forming reactions at the water/oil interface when there is a high concentration of aqueous phase to be microencapsulated. The process described by U.S. Pat. No. 3,577,515 and U.S. Pat. No. 3,575,882, while adequate if one desires to encapsulate low concentrations of water-soluble materials, is inadequate if concentrated amounts, i.e., greater than 480 grams/liter of aqueous or discontinuous phase liquid is to be encapsulated.
Surprisingly, it has been discovered that through the use of an oil-soluble alkylated polyvinylpyrrolidone (PVP) polymer one is able to form high concentration water-in-oil emulsions which are sufficiently stable to allow chemical reaction at the water/oil interface. The present invention thus provides a new and improved encapsulation process via an interfacial polycondensation reaction which is rapid and effective to encapsulate high concentrations of water-soluble materials.
The critical feature of the present invention resides in the use of the specific emulsifiers described herein to form a sufficiently stable water/oil emulsion so that a concentrated amount of water-soluble material is present in the aqueous or discontinuous phase and is thereafter encapsulated. Generally, there will be greater than 480 grams of aqueous or discontinuous phase liquid per liter of total composition. The finished microcapsules do not agglomerate nor does the capsule mass solidify when stored for extended periods of time or when exposed for short periods to elevated temperatures.
The invention is applicable to a large variety of polycondensation reactions, i.e., to many different pairs of reactants capable of interfacial condensation at the organic/aqueous phase interface to form microcapsules. A number of basic types of polycondensation reactions, are known and can be utilized in the present process. Thus, as examples, the resulting capsule skin or enclosure may be produced as a polyamide, polysulfonamide, polyester, polycarbonate, polyurethane, or polyurea, and the reactions of the invention may also involve mixtures of reactants in one or both phases, so as to yield corresponding condensation copolymers if desired, e.g., mixed polyamide/polyester, or polyamide/polyurea capsule shell walls.
The present invention is particularly advantageous when employed to encapsulate liquid fertilizers, herbicides, e.g., isopropylamine salt of N-phosphonomethylglycine, potassium salt of dicambaa and the tetramethylammonium salt of 2,4-D, plant growth regulators, insecticides, fungicides and the like.
The process of this invention is particularly advantageous if the continous phase, i.e., the organic liquid is itself a pesticide as for example, the herbicides alachlor, metalachlor, 2-chloro-N-(ethoxymethyl)-6'-ethyl-O-acetolindide, etc. Through the use of the process of this invention one is able to produce a composition containing a mixture of two or more incompatible active agents, for example, a water-soluble herbicide like the isopropylamine salt of glyphosate dispersed throughout an oil-soluble herbicide like alachlor. Formulation additives such as film forming agents can be added directly to the final suspension to improve the adhesion of the microcapsules to foliage, the dispersion of the organic phase liquid in water, etc. In some cases, reduced toxicity and extended activity of encapsulated herbicides and pesticides may result.